It's the base sourcecode of the game which you won't find in your addons.
But you need to check the lowest lines to see how the error occured and moves into the base code then:
->/AddOns/Librarian/Librarian.lua:563

Something is not correct in the code where the pressed mouse button is released above a row of the Librarian list.

This function gives data to the ESO code function "EsoUI/Ingame/LoreReader/LoreReader.lua:386: in function 'LoreReader:ApplyMedium'" and some value is not compatible here.
So one would need to check the code in file EsoUI/Ingame/LoreReader/LoreReader.lua:386 to see which variable could be nil, then go back to the librarian code and check which value is used there.

Then check which value is needed and change the value in the addon, passed to the eso code, so it fits again.

Any update on a timeline for a fix? I understand if still busy with real life, of course - just wanted to inquire since it's been more than a month.

OR...Is it working for everyone else now?

I uninstalled the mod entirely, cleaned out the savedvariables, and reinstalled - and it's still having problems...

I'm having 2 problems:

1.) the problem where books won't open and they throw the error messages listed above - but only for *some* books (Primarily those that seem to be particular to abah's landing/hew's bane/the thieves guild DLC for me so far? Weird).

* NOTE:
Yes, I *HAVE* checked the .lua and the items that people have said to change are already what they should be - so please don't re-direct me to those posts again as though that's a fix - I've already checked/done it

2.) I'm also having another problem with some books where it will save the books and tell me "684 words" or something and then I open it to read it and it's just the title of the book and the pages are otherwise empty.

Really appreciate any help you could offer in getting the mod working again - it's indispensable! Thank you so much!